In the past, the three-group zoom lens has been known as a zoom lens for various kinds of cameras. An effort has been made to miniaturize this three-group zoom lens and use it widely from the perspective of better aberration correction. For digital cameras and video cameras that have rapidly come into wide use in recent years, there has been a demand for miniaturization, higher resolution, lower distortion, and a reduced cost for this lens in the same way as similar demands for lenses used in cameras generally.
At the same time, autofocus has been the main trend for digital cameras and video cameras, and there has been demand for faster focusing speed. For this reason, the inner focus type or the rear focus type of lens, which allows lighter lenses to be made and makes the driving operation easier by keeping the lens close to the camera body, has been used quite often as the focusing method of the zoom lens. In these arrangements, with regard to the number of lens groups, a three-group construction is preferred over a two-group construction.
This type of three-group zoom lens, which adopts a rear focus method that allows faster focusing and miniaturization, that provides favorable aberration correction while achieving higher resolution has been disclosed, for example, in Japanese Laid-Open Patent Application 2000-284177 and Japanese Laid-Open Patent Application 2004-93647.
However, in recent years, there has been demand for a three-group zoom lens that provides further miniaturization and further reduced cost of the optical system. In trying to achieve these results, with the construction described in Japanese Laid-Open Patent Application 2000-284177, the space between the second lens group and the third lens group from the object side of the zoom lens remains nearly the same during zooming. Since focusing is performed by the third lens group, the space between the second lens group and the third lens group must therefore be maintained during both zooming and focusing and therefore it is difficult to shorten the length of the zoom lens at the time of retraction when the lens barrel frame construction is considered. Furthermore, because a cemented lens is not used within the second lens group, the total length of the optical system cannot be shortened at the time of retraction by the amount of an air space that a cemented lens component would eliminate.
Also, a plastic, that is, a synthetic resin, may be considered for use as a lens material in order to achieve lower cost. However, when a lens is composed of plastic, reflectance becomes larger in comparison with a lens composed of optical glass even if an anti-reflection coating is applied. Because of this, a ghost is generated by the reflected light and the image quality deteriorates.
Furthermore, when a picture is taken where there is insufficient light, it is necessary to supplement the insufficient quantity of light by amplifying the quantity of light for the wavelength region where the quantity of light is primarily insufficient. For instance, if the wavelength region where the quantity of light is insufficient is around 420 nm, there is no particular problem when the correction of the quantity of light is not performed because the sensitivity of a human eye is smaller in relation to a short wavelength region of around 420 nm. However, when the quantity of light of the corresponding short wavelength region is amplified, the chromatic aberration may sometimes be prominent. Therefore, it is necessary in such a case to correct the chromatic aberration around 420 nm.
Furthermore, such inconvenience has been difficult to resolve even with the construction described in the aforementioned Japanese Laid-Open Patent Application 2004-93647.